Adjustable ion collector slit for mass spectrometer



July 26, 1960 B. A. PETERS ETA 2, 6,88

ADJUSTABLE ION COLLECTOR SLIT FOR MASS SPECTROMETER Filed Oct. 19, 19562 Sheets-Sheet 1 ION SOURCE W 4 2 I I ION BEAM FIGJ.

INVENTORS. BELDON A. PETERS. JAMES A. RICKARD.

" ywm ATTORNEY.

July 26, 19601 B, A, PETER ETAL ADJUSTABLE ION COLLECTOR SLIT FOR MASSSPECTROMETER 2 Sheets-Sheet 2 Filed Oct. 19, 1956 ION SOURCE R O T C E LL o C N O ION SOURCE ION SOURCE hzmtznu 20- TIME.

(MAGNETIC FIELD VARYING TIME I DISPERSION 1 VARYING TIME I COLLECTORSLIT WIDTH F'IG. 9.

TIME, MAGNETIC FIELD hzummao ZO INVENTORS. BELDON A PETERS,

JAMES A. RICKAR D. BY j Z ATTORNEY.

United States Patent 6 ADJUSTABLE ION COLLECTOR SLIT FOR MASSPECTROMETER Filed Oct. 19, 1956, Ser. No. 617,019

7 Claims. (Cl. 25041.9)

This invention concerns an adjustable slit device for use in massspectrometers. More specifically, it is directed to an adjustable ioncollector slit for use in mass spectrometers.

Referring briefly to the figures wherein identical numerals designateidentical parts, I j

Fig. 1 is a sectional view of a vacuum chamber of a mass spectrometershowing the adjustable slit device arranged thereon in operativeposition;

Fig. 2 is a view taken on lines 22 of Fig. 1;

Fig. 3 is an elevation of the device of Fig. 2 with the slit adjustingplate removed;

Fig. 4 is' a view of the removed slit adjusting plate;

Figs. 5 to 7 are schematic illustrations of the operation of a massspectrometer; and

Figs. 8 and 9 are plots of ioncurrent versus time (magnetic fieldvarying time) for different slit width openings.

A mass spectrometer commonly analyzes an unknown sample by ionizing themolecules or atoms of the sample by passing the sample in gaseous formthrough an electron'beam. As seen in Fig. 5, the ion source 10 emits anion beam, the ions of which are accelerated by an electric field throughcollimating slits 11, the moving ions indicated at 12 are then passed ina vacuum through a magnet-ic field as at -13 where the ions are deviatedin different paths according to their masses. The lighter ions beingindicated by the line 14 and the heavier ions being indicated by theline '15. The ion beam designated 16 passes through a primary collectingslit 17 and' the ion current is measured in the ion collector .18 by anysuitable means. An additional secondary collecting slit, not shown, maybe employed in conjunction with the collecting slit 17. However, becausethe collimating slits 11 are not infinitely narrow and becausedefocusing effects are always present, each ion beam is not perfectlycollimated as it leaves the collimating slits 11. Hence, a beam composedof ions of only one mass would appear as shown in Fig. 6. This figurehas been enlarged and the imperfect collimation has been exaggerated forillustration purposes. If the mass spectrometer is properly constructedand the magnetic field properly shaped, the aberration of the movingions 12 after passing through collimating slits 11, as shown by thearrowed lines in Fig. 6, may be compensated for to a large extent.

As seen in Fig. 7, some of the ions are not perfectly focused andfeathering caused by imperfect focusing occurs as indicated at 19. Ifthe collection slit 17 were finite in width, although small compared tothe dispersion (the dispersion being the distance from one focus pointto the next designated 20 in Fig. 5) and if the magnetic field werevaried sufliciently slowly that each mass beam were passed through thecollection slit in turn, a graph of ion current vs. time would be asshown in Fig. 8, wherein three different masses are represented by thecurves 25, 26 and 27, respectively. The collection slit width isdesignated at 28' and the feathering Kid caused by aberration isindicated at 29. No aberration caused by imperfect focusing andinstrumental discriminations occurred, the curve of ion current vs. timewould follow the dotted lines 30, 31 and 32 for the masses representedby curves 25, 26 and '27, respectively.

. If the dispersion is large, compensation may be made by adjusting theslit width 28. For example, if the, collector slit width 28 wereincreased, the curves of the masses represented by 25, 26 and 27 in Fig.8 would be as shown by curves 35, 36 and 37 respectively in Fig. 9. Thisprocedure of adjusting the slit width to compensate for dispersion maybe used as long as the slit 'width is smaller than the dispersion.However, the dispersion changes with mass, sothat a proper slit widthfor masses of atomic mass units 44, 45 and 46, for example, may not beusable for masses of atomic mass units 144, 145 and 146. It is,therefore, desirable to be able to change the collector slit width whilethe mass spectrometer is in operation.

It is known to vary the width of collector slits in mass spectrometersby employing a magnet or iron bar and screw thread device whereby aninternal magnet or iron bar mounted in the vacuum chamber of a massspectrometer is twisted by an external magnet mounted ads jacent thevacuum chamber to thereby actuate the screw thread device which in turnadjusts the slit width. However such an arrangement is disadvantageousin many respects. For example, it is somewhat complicated to construct;there is inherent backlash in the screw threads; the apparatus may stickor be otherwise diflicult to operate due to the relatively small torquewhich can be transmitted by the magnetic fields; and the magnet may havea harmful efiect on the ion beam path. The present invention'is animprovement over such apparatus.

An object of this invention is, therefore, to provide an adjustable ioncollector slit for mass spectrometers; which has no backlash; which cantransmit considerable torque thereby preventing sticking of the slitwidth control mechanism; which does not employ a magnet; and which canprovide a continuous indication of the width of the collection slit.

Briefly, the invention comprises a device for use in the vacuum chamberof. a mass spectrometer includinga first plate member formed to providean opening therein and positioned in the path of an ion beam projectedfrom an ion source; a second plate member is slidably'arranged on thefirst plate member and is adapted to cooperate with the first platemember to vary the size of the opening upon movement thereof; resilientmeans is positioned between the first and second plate members adaptedto urge the second plate member in a direction whereby the area of theopening is enlarged; movable rodmeans is provided extending sealinglyinto the chamber and engaging the second plate adapted to move thesecond' plate against the bias of the resilient means and scale means isprovided connected to the rod adapted to indicate the size of theopening.

For a detailed description of the adjustable slit mechanism, referenceis now made to Figs. 1 through 4. In these figures is shown a vacuumchamber housing 40 in which is positioned cylindrical member 41 throughwhich an ion beam is projected from an ion source, designated 42. Acylindrical plate member 43 is secured to an end of cylindrical member41 by means of screws 44 and a positioning pin 44. to provide an opening45 and is also formed to provide an indented portion 46 extending fromone side of opening 45 to the periphery of plate 43. Additional platemembers 47, as seen more clearly in Fig. 2, are secured to plate member43 by means of screws 48. The plate members 47 are spaced apart as shownand provide slots or trackways adjacent the indented portion 46 of plateThe plate 43 is formed 43. A plate member 50is slidably arranged in theindented portion of plate 43 on protuberances 50 which are slidablyarranged in the slots formed by plates 47. Plate member 50 cooperateswith opening 45 in plate member 43 to form an adjustable slit opening45'. A flange 51 is-formed on plate member 50- and a spring member 52 isarranged betweenplate member 4-3 and plate member 50, as-shown. Each endof spring member 52 ispositioned between one of the plates 47 and plate43 and the center engages flange 51 thereby biasing: the plate member 50away from plate member 43; that is, in a direction-whereby the slit 45is widened. The inner end ofa push rod 53 engages flange 51 and extendsthrough the wall of the vacuum chamber 40. A third plate member 65 is:positionedbetween plate member 43 and an ion'collector 66 and is formedto provide an opening 67 arranged in alignment with opening 45'. Platemember 65 is secured-to plate member 43 by means of bolts 68; Platemember 65' is used in a well known manner and does not form a part ofthis invention.

A bellows 54, as seen in Fig. l, is secured and hermetically sealedtoone end of push rod 53 as at 60. The other end of bellows 54 is securedand hermetically sealed about an opening 61 in the wall of housing it?as seen at 617. The push rod 53 is movable into and out of housing 40.through opening 61. The bellows 54 and push rod 53 are arranged in ahousing 56 which is connected to the exterior wall of chamber 40 bymeans of bolts 58. A micrometer having a micrometer head 57 providedwith a scale indicator, not shown, is connected to housing 56. Themicrometer head 57 extends through housing 56 and'engages with the outerend of push rod 53;.

In operation when it is desired to narrow slit opening 45 the micrometeris rotated in one direction to move pushrod. 53 against the bias ofspring 52, thereby narrowing the width of slit opening 45'; on the otherhand, when it is desired to Widen slit opening 45', the microm eter isrotated in a reverse direction'to mo ve push rod 53. away from theflange 51 permitting spring 52 to move plate member 50 in the directionof movement of the push rod 53, thereby enlarging the width of slitopening 45. Thus, the ion beam projected from ion source 42 throughopening 45 may be adjustably transmitted through opening 45 and thentransmitted to the ion collector 66 through opening 67 in plate member65. By having the micrometer properly zeroed, the width of the slitopening 45 may be continuously indicated for any selected positions ofplate 50.

Having fully described the structure, objects and operation of ourinvention, .we claim:

1. Adevice for forming an adjustable ion collector slit in a massspectrometer comprising a housing, a support mounted in said housing, afirst plate member secured to said support, a second plate memberslidably arranged on said first plate, said first and second platemembers cooperating to form a slit opening, the width of said slitopening being varied upon movement of said second plate member,resilient means positioned between said first and second plate membersadapted to urge said second plate member in one direction, movable meansextending sealingly through said housinga-nd-engaging said second platemember adapted to urge said second plate member against the bias of saidspring and means engaging with said movable means adapted to indicatethe width of said slit opening.

2. A device as recited in claim 1 wherein said movable means is a rodand said means engaging therewith is a micrometer head.

3. A device for forming an adjustable ion collector slit in a vacuumchamber of a mass spectrometer comprising a first plate member formed toprovide a first opening arranged in said chamber in the path of an ionbeam, a second plate member slidably' arranged on said first platemember adapted to cooperate with said first plate member to form anadjustable second opening, resilient means positioned between said firstand second plate members adapted to bias said second plate member in adirection whereby said second opening is enlarged, movable meansextending sealingly into said chamber and engaging said second platemember adapted to move said second plate member against the bias of saidresilient means and scale means connected to said movable meansadaptedto continuously indicate the size of said second opening.

4; A device as recited in claim 3 wherein said movable means is a rodand said-scale means. is a micrometer. head.

5. A device for forming an adjustable ion collector slit in a vacuumchamber of a mass spectrometer comprising a first plate member formed toprovide. an opening arranged in said chamber in the path of anion beam,a second plate member slidably arranged on said first plate memberadapted to cooperate with said opening to vary the amount of said ionbeam passed through said opening, resilient means positioned betweensaid first and second plate members adapted to bias said second platemember in one direction, movable means extending sealingly into saidchamber adapted to move. said second plate member against the bias ofsaid resilient means, and means positioned exterior of said chamber toindicate the amount of said ion beam being passed through said opening.

6. A device as recited in. claim 5 wherein said movable means is a rodand. said exterior means is a micrometer.

7. A device as recited in claim5 wherein a bellows seals olf fluidcommunication between said exterior means and said chamber.

References Cited in the file of this patent UNITED. STATES PATENTS2,292,087 Ramo Aug. 4, 1942 2,378,962. Washburn June 26, 1945 2,674,698Dmorth et a1 Apr. 6, 1954 2,709,222 Lawrence May 24, 1955 2,852,684Payne Sept. 16, 1958

